Electron multiplier



Feb. 11, 1941. R 'SNYDER JR 2,231,691

' `ELECTRON MULTIPLIER Filed Nov. 50, 1937 Snnentor mmgdy Patented Feb. 11, 1941 PATENT OFFICE ELECTRON Richard L. Snyder, Jr.,

MULTIPLIER Glassboro, N. J., assignor to Radio Corporation of America., a, corporation of Delaware Application November so, 1937, serial No. 177,286

4 Claims.

My invention relates to electric discharge tubes, particularly to electron multipliers, and has special reference to the provision of improvements in multi-stage electron multipliers Wherein the electron stream may 4be of such intensity as to initiate a disturbing iiow of ions in the residual gas.

One trouble encountered in the operation of multi-stage electron multipliers is that when the electron stream has been increased by several stages of electron multiplication, positive ions may be generated in the residual gas. This phenomenon has been observed to eXist even in the most thoroughly evacuated envelopes. These positively charged particles ow in a direction opposite to the general direction of the electron stream and may impinge upon the primary-electron emitting cathode, or upon an intermediate secondary-electron emitter, Where they release still more electrons. The electron current may thus be augmented to such a degree as to render it uncontrollable. This building up of the electron current is limited only by the so-called space charge eect and may result in an' equilibrium of the output current, i. e., the output current may no longer accurately reflect variations in the input current.

Rajchman and Pike, in their copending application Serial No. 171,916, led October 30, lf`37,describe an open sided electrode assembly for electron multipliers wherein each emitter electrode has a surface which extends into the space between `the cathode and anode whereby these several surfaces constitute a barrier to the passage of ions in the anode-cathode directions. Such construction, indeed, bars or entraps and dissipates the vast majority of the ions adjacent their area of origin and thereby ensures improved performance. The improved performance is best -evidenced by the fact that the output current is directly proportional to the input current substantially irrespective of the intensity of the electron stream impinging upon the output electrode. It has been observed, however, that when the Rajchman and Pike tubes are operated at or close to their maximum output, there is a tendency for the tube to become noisy I have discovered that this undesired operating characteristic may be attributed to the escape of some ions from the ion traps due to the distorted contour of the equipotential lines adjacent the lateral edges of the open sided electrode assembly. As a result of numerous observations, I conclude that these escaped ions flow (Cl. Z50-175) to the low current (input) end of the device, where they apparently impinge upon the inner surface of the container, thus creating a positive charge thereon. When the positive charge on the tube wall extends to the area which surrounds the cathode, the, positive surface will draw some of the emitted primary or photo-electrons thereto. These negatively charged particles neutralize the positive charge on the glass Wall and in so doing create an electrical disturbance within the tube. Such disturbance Will ordinarily be of a continuing nature because the positive charge having been neutralized, the escaping ions will again be drawn thereto, followed later by the electrons, from the cathode, which provide the disturbance incident tothe described neutralizing action.

It would appear that the disturbing effects incident to the escape of ions from the open sides of the Rajchman and Pike electrode assembly. might be obviated simply by enclosing the assembly, for example, with mica walls, in the manner shown in U. S. Patent 2,113,378 to Massa et al. rSuch mode of construction is effective in preventing the escape of ions, providing the structure is perfectly tight, but I have found that optimum performance is not achieved in electrostatic type multipliers (though it may be in electromagnetic type multipliers) when the electrode assembly is entirely enclosed, principally because the enclosing walls alter the contour of the electrostatic elds which provide the electron focusing action.

Accordingly, an object of my invention is to provide an electrostatic type electron multiplier the operation of which is characterized by substantial freedom from those spurious internal electrical and electro-mechanical vibrations Whichuconstitute or give rise to tube noise.

Another object of my invention is to achieve a, noise-free electrostatic type electron multiplier without any sacrice in the efliciency or change in contour of the electron focusing elds.

Another object of my invention is to provide an electron multiplier wherein dark current (i. e., current Which flows when the tube is energized but the cathode not illuminated) is substantially obviated.

Other objects and' advantages, together With certain details of construction, will be apparent and the invention itself will be best understood Iby reference to the following specification and to the accompanying drawing, wherein:

Figure 1 is a longitudinal sectional view of an electron multiplier constructed in accordance with the principle of the invention,

Figure 2 is a cross-sectional view taken on the line 2-2 of Fig. 1, and

Figure 3 is a top plan View of the device of Fig. 1.

Like reference characters designate the same parts in all gures.

In the drawing, T designates a highly evacuated envelope which is preferably, though not necessarily, in the form of a cylindrical glass tube having a long central axis indicated by the dotted line :0. The envelope or tube T is provided with a press P which is preferably circular, whereby it may accommodate several (12 in this case) electrode leads L without undue crowding. If desired, these leads L may be rigid enough and strong enough to support the electrodes to which they are connected. However, in this case, a number of disc-like spacers D, provided with springs S, which contact the inner wall of the tube, contribute to the support of the latter described electrode assembly. The several leads L are preferably provided with insulation such, for example, as pieces of glass tubing G, of small diameter, which extend through the discs D to prevent short-circuiting. The discs or spacers D are cut away adjacent their central portions and provide a support for an insulating frame F, which comprises upper and lower strips F1, F2, respectively, of glass or the like, which are maintained in spaced relation as by end-pieces F3, F4 of similar insulating material.

In the illustrated embodiment of the invention, the rigid insulating strips F1, F2 are of undulate formation, the undulations of each strip being uniformly spaced along the axis or median line :1: with the convex sections of one strip drectly opposite the concave sections of the 0p- DOSite strip. Seated within each undulate con- Electrode I is a photosensitive cathode.

cave section of the frame F is a curved electronemissive electrode plate; those on the lower strip F2 are numbered I, 3, 5, 1, 9 and II, and those on the upper strip 2, 4, 6, 8, I0 and I2.

It is provided with a return bend portion I1' which is of foraminous construction to permit light from an external source, exemplified in Fig. 1 by the lamp Y and lens Z, to impinge upon the photosensitive surface of the cathode section Ip. 'I'he intermediate electrodes 2 to II inclusive are the multiplying electrodes. They are formed, for eX- ample, of silver suitably coated with a substance which is the equivalent of caesium, to render them capable of a copious flow of secondary electrons. The electrode nearest press P is the electron collecting electrode, or anode; it is here shown in the form of an inclined imperforate plate I2 supported at the top by a curved plate I2a (which is conveniently similar to electrodes I to II), whose edge extends slightly beyond the inclined collecting surface.

Electrodes I to II inclusive are preferably of L-shape contour and, as described in the Rajchman and Pike disclosure, have their long legs m inclined toward the cathode with the transverse terminal edges of these long legs extending to, or slightly beyond, equally spaced points along the central axis :c of the tube. The short legs n of these L-shaped electrodes extend in the general direction of the collector electrode or anode I2 and terminate at points removed substantially equal distances from the said axis m. The long leg m of each L-shaped multiplying electrode thus extends beyond the short leg n of the next preceding L-shaped electrode of the same set. This construction intensifies the electrostatic eld in the cup of the next preceding electrode of the same set and thereby provides a higher effective emission or saturation of the secondary electron emission from that electrode. A further result of this construction is that the majority of the ions generated in the residual gas by the passage of the electron beam will be promptly arrested by the intruding long legs m of the multiplying electrodes. Thus, the majority of the ions generated in the space between the multiplying electrode Il and the anode I2 will be collected by the long legs m of electrodes I l and I0.

'The contour and general arrangement of the electrodesabove described are similar in most respects to that of the Rajchman and Pike disclosure. The insulating frame and its support, however, are within the purview of the present invention. The disc-like members D serve not only as elements of a non-microphonic support for the electrode assembly, but also serve to prevent the llow of ions which may escape through the open sides of the electrode assembly. The discs serve in two ways to prevent the ow of ions. First, they act as physical barriers to the passage of escaped ions and, secondthey provide means for electrically extending the equipotential lines of force between adjacent electrodes out and beyond the edges of the said electrodes whereby ions generated in the space between these electrodes are constrained to remain Within or adjacent their area of origin instead of being drawn outwardly by the distorted field usually present adjacent the side edges of the multiplying electrodes.

In order to achieve the desired extension of the electrostatic eld beyond the side edges of the electron conduit in which the disturbing ions are generated, the present invention contemplates the use of one or more pairs of auxiliary conductive elements or baliles in the space be- .tween each of the side edges of the conduit and the inner wall of the tube.

In the embodiment of the invention shown in the drawing, three pairs of baliles d1J d2, d3 are provided. Asl shown, these elements may be supported upon, and electrically connected to, the correspondingly numbered discs D. They are further connected, respectively, to an adjacent multiplying electrode in either the top or bottom set. Thus, as more clearly shown in Fig. 2, elements d1 are connected, as by short leads w, to both the bottom multiplying electrode 3 and to disc D1. Neither discs Dl nor part d1, however, contacts electrode 4 in the upper set, so that short circuiting is avoided. Elements d3 Will be understood to be similarly connected to disc D3, and to electrode 9, while baffles d2 are connected to multiplying electrode 6, in the upper set, through disc D2- Each -baffle d is preferably in the form of a bent metal strip having a part a which follows the contour of the long leg m of the multiplying electrode to which it is connected, to a point adjacent its transverse edge, and a part b which bends backwardly (i. e., in the general direction of the anode) and toward the transverse edge of the next preceding (in point of electron travel) multiplying electrode in the opposite upper or lower) set. Since each baille is connected (by short leads w) to a multiplying electrode, the bent part b of each baille will project an electric field or surface into the space about the side edges of the electrode assembly, and the potential on this virtual surface will be the same at every point as that on the electrode to which the baiiies are connected. I attribute the improved operating characteristics of these tubes, at least in part, to the fact that by extending the electrostatic field beyond the edges of the conduit through which the electrons pass (and in which ions are generated), there is no substantial distortion in the eld adjacent these edges so that the electrons and ions are constrained to travel in more or less straight lines along the axis (rv) of the tube instead of being urged outwardly toward and beyond the edges of the conduit.

In the illustrated embodiment of the invention, but three sets of baiiles are shown. If necessary or desirable, a pair of baflies may be provided for each pair (upper and lower) of multiplying electrodes. It would needlessly complicate the drawing to show such modification.

While the invention has been described in connection with an electrostatic type electron-multiplier similar to that described in the above identified Rajchman and Pike application, it is to be understood that my invention is not limited thereto as the disclosure is, in this respect, merely illustrative for purposes of explaining the inventive concept.

What is claimed is:

1. An electric discharge device comprising an evacuated envelope containing a small amount of residual gas in which ions may be generated, said envelope having a long axis, a. cathode and an anode mounted adjacent opposite ends of said axis, a plurality of sets of multiplying electrodes mounted on opposite sides of said axis intermediate the cathode and anode, the side edges of said multiplying electrodes defining an open sided electron conduit from which ions may escape when said residual gas is bombarded by electrons from said cathode and multiplying electrodes, and means disposed adjacent the open sides of said electron conduit and extending into the space intermediate said conduit and the inner wall of said envelope for barring the passagey of said escaped ions.

2. An electric discharge device comprising an evacuated envelope having a long axis, a cathode and an anode mounted adjacent opposite ends of said axis, multiplying electrodes mounted on opposite sides of said axis between said cathode and anode, said electrodes constituting an open sided electrode assembly, and a conductor mounted exterior of said assembly `and extending across Aan open side thereof and connected to one of said multiplying electrodes, said conductor having a contour which corresponds substantially to the contour of the equipotential line projected, when the device is energized, from the electrode to which it is connected.

3. An electric discharge device comprising an open sided electrode assembly mounted within an evacuated envelope, said envelope containing a residual gas in which ions may be generated when the device is energized, a disc disposed intermediate the ends of said electrode assembly and supported adjacent its periphery on the inner wall of said envelope, said discvhaving a central aperture which extends substantially across said envelope and in which said electrode assembly is supported,l and a pair of bailies supported on opposite sides of said disc between the wall of said envelope and the open sides of said electrode assembly, said baliles and the surface of said disc constituting a barrier to the passage of said ions.

4. The invention as set forth in claim 3 wherein said baiiles are connected to a common electrode and serve to extend the electric field about said electrode, when said device is energized, beyond the open sides of said electrode assembly.

RICHARD L. SNYDER, JR. 

